The invention relates generally to the field of measurements of subsurface formation properties using pulsed neutrons as an energy source. More specifically, the invention relates to determining fractional volume of pore space (porosity) of subsurface formations using a pulsed neutron source and at least one gamma ray radiation detectors.
Subsurface formation HI (hydrogen index) measurement using high energy neutrons as a measurement source has been used in well logging since at least the 1950s. In case of no bound-water in the formation matrix (such as shale), hydrogen atoms only appear in the pore space (oil or water). Thus, the formation hydrogen index is typically a measurement of formation porosity. Neutron source based porosity measurements known in the art rely on the fact that the slowing down of neutrons, and therefore the average distance travelled within the formations by the neutrons, is strongly dependent on the hydrogen content of the formation. The hydrogen content dependency is due to the fact that neutrons can incur a very large energy loss in a single elastic scattering event with a proton (a hydrogen nucleus). In its simplest form, neutron based porosity measurement can be performed using a neutron source and a detector axially spaced from the neutron source. If the axial spacing of the detector from the source is chosen appropriately, then the neutron flux at the detector location will decrease monotonically with increasing formation hydrogen content. As one possible alternative, the neutron detector can be replaced by a gamma-ray detector, since the flux of neutron induced gamma-rays is related to the neutron flux.
Early versions of neutron-based porosity measurement instruments included those having a single gamma-ray detector (e.g., a Geiger-Mueller counter) with a radioisotope-based neutron source (e.g., 241AmBe, 238PuBe). Such instruments may be referred to as “neutron-gamma” instruments. Correspondingly, instruments using a neutron detector (e.g., a 3He proportional counter) may be referred to as “neutron-neutron” instruments. Traditionally, the term “neutron porosity” typically means a neutron-based porosity measurement using a 241AmBe source and “neutron-neutron” instruments. The following terms are defined in order to differentiate this work from the traditional “neutron porosity”. “Neutron-neutron porosity” may be defined as neutron porosity based on a neutron source and neutron detectors. Similarly, “neutron-gamma porosity” may be defined as neutron porosity based on a neutron source and gamma ray detectors, which is the scope of this invention.
Both neutron-neutron instrument measurements and to an even larger extent neutron-gamma instrument measurements are strongly affected by a multitude of environmental effects. Most, if not all open-hole neutron tools used at the present time are neutron-neutron measurements based on the detection of the thermal and/or epithermal neutron flux at one or more neutron detectors.
It can be more difficult to measure formation HI based on gamma ray detectors as compared to using neutron detectors. In addition to other phenomena, gamma ray detectors measure the gamma rays from neutron “capture” interaction (i.e., capture of a thermal neutron by a nucleus of certain atoms having large “neutron capture cross section” and subsequent emission of a gamma ray) in the formation, wellbore or the instrument itself. Capture gamma ray measurement is therefore an indirect measurement the presence of neutrons. The physics of neutron-neutron porosity only involves neutron transport from the source to the neutron detector. The physics of neutron gamma porosity involves both neutron and gamma ray transport, so that such physics are more complex. Thus, neutron-gamma porosity may have more environmental effects which may be more difficult to interpret.
Notwithstanding the additional complexity in interpretation there may be advantages associated with measuring neutron-gamma porosity. The count rate of a gamma ray detector can be more than 1 order of magnitude higher than a 3He neutron detector. The depth of investigation (lateral distance from the wellbore wall into the formation) of a neutron-gamma measurement may be deeper than that of a neutron-neutron measurement. The energy of a gamma ray from a neutron capture event is normally in the million electron volt (MeV) range, which means such gamma rays can travel a longer distance than a thermal neutron before absorption. A scintillation type gamma ray detector can also provide gamma ray spectroscopy and inelastic neutron scatter-related measurements, which a thermal or epithermal neutron detector cannot. The foregoing features make neutron-gamma porosity very appealing. Thus is it desirable to have an improved neutron-gamma porosity instrument.